Field
This application relates generally to a craft-mounted tunable antenna assembly.
Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
Antennas vary greatly in design and configuration. Some antennas extend through multiple planes, whereas other antennas extend through a single plane. Antennas that are secured to moving vehicles or craft are required to extend through as small of a space as possible. These low-profile antennas have poor gain and poor efficiencies. Tuning an antenna may mitigate some of the poor performance gain, however, the efficiencies in these low-profile antennas are still very small. This is due to the size requirements or limitations on the antenna when it is secured to a vehicle or craft.
Tunable antennas have been previously achieved by use of PIN diode switches and serial inductances from helical coil line lengths. These designs are inefficient because the number of PIN diodes is large, resulting in a large amount of power, poor antenna performance, e.g., gain and reception inefficiencies.
U.S. Pat. No. 6,466,169 discloses a planar serpentine slot antenna. The antenna includes two conductors, wherein the first conductor extends through a planar serpentine shape and a second conductor is etched with a comb-like configuration interleaving the serpentine configuration of the first conductor. A coaxial cable has a first conductive portion connected to the first conductor and a second conductive portion connected to the second conductor if the antenna. This planar serpentine slot antenna has limited tunability.
U.S. Pat. No. 8,325,097 discloses a tunable antenna having a variable reactance network capable of adjusting the reactance of the network to maximize the RF voltage. Although lump variable reactance can be used to improve antenna impedance matching or maximize power transfer, such technique is widely known to be limited to a certain achievable Bode-Fano theoretical bandwidth. This means that variable reactance network technique can only achieve impedance matching to a limited frequency band only. For example, this antenna uses a tunable lump capacitor to maximize the voltage. This lumping element approach to tuning cannot be tuned favorably. This is because the series lump capacitor can only reduce the capacitance values at the antenna RF port and the antenna reactance or tuning would still be poor. In other words, this series lump capacitor cannot reduce the antenna's original large reactance. Furthermore, any shunt lump capacitor added at the input RF port will not be effective either. The shunt lump capacitor would reduce the antenna reactance, but it would also adversely reduce the antenna radiation resistance and would worsen the antenna performance. In short, any variable reactance network can only improve the antenna impedance matching to a certain extent for a given antenna reactance to resistance ratio prior to any variable reactance network insertions. This antenna reactance to resistance ratio is only related to the antenna radiation structure, widely known as the Chu bandwidth limit.